Polls show Nicola Sturgeon’s nationalist party could be Scotland’s biggest winner in U.K. election; setback for Ed Miliband’s Labour

GLASGOW—As the U.K. heads toward the most unpredictable national election in a generation, one thing seems sure: The Scottish National Party is headed for a landslide in the north that could upend politics for the whole country.

The party that led last year’s failed vote in Scotland on secession has seen its support surge among Scots still fired up by the thought of independence. Its new leader, Nicola Sturgeon, has emerged as a star of this year’s campaign, outshining her mostly male opponents in televised debates with her...